Since cerium oxide has a low oxidation-reduction potential of about 1.6 V between Ce4+ and Ce3+ and hence the reaction represented by the formula below proceeds reversibly, cerium oxide has an oxygen storage capacity (OSC) and is used due to this property as a promoter or a catalyst support in an automotive three-way catalyst.CeO2CeO2−x+X/2O2 (X=0 to 0.5)
However, pure cerium oxide is well known to have an extremely low OSC of about X=0.005.
To improve this, there have been numerous reports that proposed that, by forming a solid solution of zirconium oxide in cerium oxide, (1) the heat resistance of the specific surface area of cerium oxide can be improved, (2) the OSC can be improved by inserting Zr4+ having a small ionic radius into a Ce backbone to alleviate the increase in volume during the above-mentioned reaction, and addition of a third component leads to higher performance, etc.
The air-fuel ratio in automotive gasoline engines is electronically controlled for compete combustion in order to minimize air pollution caused by nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbon (HC), an unburned component of gasoline, generated in the exhaust gas.
As a result, the oxygen concentration in the atmosphere fluctuates, i.e., there is more air than fuel (oxidative atmosphere) or more fuel than air (reducing atmosphere). Under such circumstances, cerium absorbs oxygen in an oxidative atmosphere and releases oxygen in a reducing atmosphere, thereby functioning to facilitate the oxidation reaction and the reduction reaction.
However, these reactions are repeated at high temperatures. It has been revealed that, for this reason, the volume fluctuation due to the cerium valency changes in response to the atmospheric changes distorts the crystal structure of a mixed oxide used as a promoter or a catalyst support to cause impaired stability, facilitating sintering of a precious metal used as a catalyst, thereby greatly affecting catalytic performance. Catalytic performances are known to be affected more in reducing atmosphere than in an oxidative atmosphere.
Considering the above, zirconia-ceria-based mixed oxides used as promoters or catalyst supports in automotive three-way catalysts are required to have not only standard heat resistance but also heat resistance against high temperatures under a reducing atmosphere. More specifically, zirconia-ceria-based mixed oxides capable of maintaining a stable crystal structure at high temperatures under a reducing atmosphere are demanded.
Japanese Patent No. 3623517 discloses a “composition composed of cerium oxide and zirconium oxide which has an atomic ratio of cerium/zirconium of at least 1, and which has a specific surface area of at least 35 m2/g after being calcined for 6 hours at 900° C. and an oxygen storage capacity of at least 1.5 ml/g O2 at 400° C.”, and “the composition being represented by the formula CexZryYzO2 wherein z ranges from 0 to 0.3; when z is 0, x ranges from 0.5 to 0.95; x and y are such that x+y=1; when z is greater than 0, x/y ratio ranges from 1 to 19; and x, y and z are such that x+Y+z=1.”
Japanese Unexamined Patent Publication No. 2000-176282 discloses “a catalyst for purifying a lean exhaust gas comprising:
i) an oxide solid solution containing a ceria-zirconia solid solution in which zirconium oxide is dissolved in cerium oxide, wherein the solid solubility of the zirconium oxide in the cerium oxide is 50% or higher, the average diameter of crystallite is 10 nm or less, the zirconium ratio is in a range of 0.55≦Zr/(Ce+Zr)≦0.90 on a molar ratio basis, and an oxide of a rare earth element M other than cerium is further contained in a composition range of 0.03≦M/(Ce+Zr+M)≦0.15 on a molar ratio basis;
ii) a fire-resistant porous substance; and
iii) a catalytic precious metal supported on the oxide solid solution and/or the porous substance.”
Further, Japanese Unexamined Patent Publication No. 2000-169148 discloses “a cerium-based mixed oxide represented by the following general formula, Ce1−(a+b)ZraYbO2−b/2 wherein 0.06≦b/a≦0.27.”
However, Japanese Patent No. 3623517 merely describes “the specific surface area and OSC (oxygen storage capability) at the time of high temperature calcination”, Japanese Unexamined Patent Publication No. 2000-176282 describes “the OSC (oxygen storage capacity) after durability tests”, and Japanese Unexamined Patent Publication No. 2000-169148 describes “the OSC (oxygen storage capacity) after 5, 50 and 500 hours of durability tests in high temperature air of 1000° C.”, and no description regarding “the stability of the crystal structure at high temperatures under a reducing atmosphere” is found in any of these documents.
Japanese Unexamined Patent Publication No. 2003-277059 discloses “a ceria-zirconia-based mixed oxide comprising CeO2, ZrO2, and an oxide of at least one additional element selected from the group consisting of rare earth elements, alkaline earth elements, and transition elements; and having a regular phase in which cerium ions and zirconium ions are regular arranged.
Japanese Unexamined Patent Publication No. 2003-277059 contains descriptions regarding “the ceria-zirconia-based mixed oxide having a high oxygen storage capacity (hereinafter referred to as OSC) and in which the phases are prevented from becoming separated under an oxidative atmosphere at 1000° C. or higher” and “Example 2 shows an x-ray diffraction chart and OSC measurement results of a powdered mixed oxide obtained by reducing the powdered oxide prepared in Example 1 for 5 hours at 1200° C. in a CO stream, treating the resultant for 5 hours at 1200° C. in the air, reducing the resultant for 5 hours at 1200° C. in a CO stream, and further treating the resultant for 1 hour at 500° C. in the air”; however, no description regarding “the stability of the crystal structure at high temperatures under a reducing atmosphere” is found.